The invention relates to an apparatus for intermediate enrichment of trace substances from a gas stream in a cold trap which carries a flow and optionally contains an adsorbent material and which, during the enrichment phase, is immersed into cooling fluid in a cooling bath and, for desorption, is subjected to (optionally heated) air or gas, and to a chromatography arrangement provided therewith.
The detection of trace substances in gases, in particular in air, is gaining in importance in view of the increasing pollution of the environment with the most diverse trace substances, some of which are highly toxic or highly harmful to ecology and climatology. The sensitivity of the measuring instruments for the direct detection is here frequently inadequate, so that an intermediate enrichment of the trace substances in a cold trap becomes necessary, which cold trap is immersed into a cooling fluid, from which the cold trap is taken for the actual detection (usually by chromatographic means) with heating.
The applicant has also already provided an apparatus (German Patent No. 3,729,374), in which a U-shaped cold trap surrounded by a heat-insulated container, which is open at the bottom, is immersed into liquified gas as the coolant, which is introduced into the container during the cooling phase, whereas the cold trap is heated up for desorption, coolant being displaced from the container at the same time.
Usually, the coolant used here is liquified nitrogen or liquified air, and the heating-up of the cold trap is carried out especially by electric heating of the cold trap wall consisting of a conductive material.
The detection of particularly small trace substance contents of relatively unstable compounds such as, for example, peroxyacetyl nitrate (PAN), with the aid of such intermediate enrichment has the disadvantages that, at the relatively low temperatures of the liquified gases, considerable quantities of concomitant substances, which can complicate the detection of trace substances, are collected in the cold trap and, that there is a risk of sensitive substances being decomposed by the direct heating of the cold trap.